I Reset to Factory Settings as you suggested. I can now lower the volume, even all of the way to zero using this setting by entering a negative value. But I can't increase the volume with the Change By setting. If I enter a positive value, e.g., 10, 40, even 127, it has no effect on the volume. But if I enter a negative value I can lower the volume and in fact take the volume all of the way down to zero by entering -127. I've tried starting with the volume at different levels from 0 on up and I always get this same response. Positive values do not increase the volume.

By the way, now the chords displayed on my chord sheet are in a much smaller font size and I can't find a way to change that. It's not a big deal but a curious result of resetting to factory settings.